The present invention concerns a method of forming a pattern and, more in particular, it relates to a method of forming a fine pattern suitable for the production of semiconductor devices, magnetic bubble memories, superconductivity devices and surface acoustic wave devices.
In a projection printing system of irradiating a light by way of a mask having a pattern to be formed (hereinafter referred to as a reticle) and transferring the pattern on the reticle to a photoresist film formed on a wafer, it has been demanded to make the pattern formed on the photoresist film finer. As a measure for indicating the performance of the projection printing system as to the capability of forming a fine pattern, "resolution" has been used generally. The resolution is expressed by a minimum distance between two adjacent points or lines that can allow them to be separated from each other and the resolution of existent reduction projection printing systems is about 0.5 .mu.m.
As a means for improving the resolution, it has been known to provide exposure light passing through two adjacent permeable portions of a reticle with a phase shift. A reticle pattern used for providing the exposure light with the phase shift is described in, for example, Japanese Patent Laid-Open Hei 2-078216. In this case, a transparent thin film for changing the phase oft transmission light by 180.degree. is used as a phase shifter, which is disposed at a portion of the permeable portion of the reticle. Since the presence/absence of the shifter causes a 180.degree. phase shift to the transmission light, a complete dark portion is formed along the rim of the shifter and the line width of the dark portion is extremely fine. Accordingly, an extremely fine line pattern can be formed if a positive type resist is used and an extremely fine space pattern can be formed if a negative type resist is used.
However, the above-mentioned prior art involves a problem in that patterns having various line widths can not be formed simultaneously although an extremely fine pattern can be formed. For instance, a space pattern of 0.2 .mu.m width can be formed by using the prior art described above, but it is not possible to form a space pattern, for example, of 0.2 .mu.m width, together with a space pattern of 0.3 .mu.m width or 0.35 .mu.m width. If the phase shifting mask technology of the prior art is combined with usual method of using a Cr film as a mask, it is possible to form, for example, a space pattern off 0.2 .mu.m width by the phase shifting mask technology and a pattern of greater than 0.5 .mu.m width by a usual exposing method using the Cr film respectively, but a pattern of a width between 0.2 to 0.5 .mu.m can not be obtained. In addition, there is a problem that the pattern that can be formed by the foregoing prior art is restricted to space patterns or line patterns and this method is not effective for the formation of dot patterns or hole patterns.